The present invention relates to a wire dot print head, and more particularly to a wire dot print head provided with a metallic residual sheet.
Impact printers are known in which, according to print information, print wires are driven so that the tips of the print wires are pressed against the print medium to effect printing. In such an impact printer, a wire dot print head of the plunger type, of the spring charge type, of the clapper type or the like is used.
FIG. 1 is a sectional view showing an example of a prior art spring charge type print head. In FIG. 1, print wires 1 are fixed to tips of armatures 2 and are moved toward and away from a platen PL. When the print wires 1 are moved forward, i.e., toward the platen PL, the tips 1a of the wires press the ink ribbon IR against a printing paper PP passing over the platen PL, thereby printing dots on the printing paper PP.
It is noted here that the terms "forward" and "front" in connection with the print head are used to mean "toward the platen" and "the side facing or closer to the platen". The terms "rearward" and "rear" are used to mean "away from the platen" and "the side facing opposite to or farther from the platen".
The armatures 2 are disposed between radial parts 4a of a front yoke which also has an annular part 4b to which the outer ends of the radial parts 4a are connected. The armatures 2 are mounted to inner ends 3c of radial parts 3a of a biasing plate spring 3, having its annular base part 3b clamped between an annular spacer 5 and an annular hinge plate 6, so as to form respective swinging members. The armatures 2 are normally magnetically attracted to cores 9. The magnetic attraction force is generated in the core 9 due to a magnetic flux generated by an annular permanent magnet 7 and passing through a generally cup-shaped rear yoke 8 comprising a disk-shaped base 8b and a cylindrical side wall 8c.
Each of the radial parts 3a of the plate spring 3 functions as a plate spring independently of the other radial parts, and so each radial part 3a can be referred to as a plate spring.
The cylindrical side wall 8c, the annular permanent magnet 8, the annular hinge plate 6, the annular part 3b of the plate spring 3, the annular spacer 5, and the annular part 4b of the front yoke 4 form an annular wall of the print head, while the disk-shaped base 8b of the rear yoke 8 forms the bottom wall of the print head.
A guide member comprises a flange part 40a connected to the annular part 4b, and a nose part 40b provided with guide conduit 40c with notches 40d at which wire guides 16 are received. Each of the wire guides 16 has holes through which the print wires 1 are passed such that they are slidably guided for movement toward and away from the platen PL.
Cores 9 extend from the front surface 8d of the disk-shaped base 8b of the rear yoke 8. Fulcrum members 8a also extend from the front surface 8d of the disk-shaped base 8b of the rear yoke 8. As is better illustrated in FIG. 2, the fulcrum members 8a are provided in association with respective cores 9 and are disposed between the associated cores 9 and the annular wall of the print head.
A demagnetizing coil 10 is wound on each of the cores 9 to form an electromagnet, and when a demagnetizing current is made to flow through the demagnetizing coil 10, the electromagnet generates a magnetic flux canceling the magnetic flux due to the permanent magnet 7. Because the magnetic force, which attracts the armature 2 to the core 9 thereby bending the radial part 3a, diminishes, the swinging member swings forward moving the armature 2 toward the platen PL because of the resilient force of the plate spring. Due to the swinging, the print wire 1 moves forward of the print head, being guided by the wire guides 16, and strikes the ink ribbon IR and the printing paper PP to effect printing.
Energization of the demagnetizing coil 10 is terminated at an appropriate time, and a rebounding force caused by the impact on the platen PL acts on the print wire 1. Accordingly, the print wire 1 begins to return backward, i.e., away from the platen PL. Due to the magnetic flux from the permanent magnet 7, the armature 2 is again magnetically attracted to the core 9, and a printing operation of one cycle is thus completed.
The swinging motion of the swinging member made about the tip 8e of the fulcrum member 8a, and so sliding friction is generated at the tip 8e. At the time of returning, the armature 2 collides, and so wear due to collision occurs on the core 9. In order to the prevent the wear, a partition sheet including a circular metallic residual sheet 13 is inserted between the core 9 and the plate spring 3, as shown in FIG. 1. Moreover, a front plastic film 14 is inserted between the metallic residual sheet and the radial parts 3a. More specifically, as shown in FIG. 4, an exploded view, the front plastic film 14 is circular, has the same diameter as the metallic residual sheet 13, and is superposed with the metallic residual sheet 13 to cover the entirety of the front surface of the metallic residual sheet 13. A rear plastic film 15 is inserted between the metallic residual sheet 13 and the tips 8e of the fulcrum members 8a. More specifically, as shown in FIG. 3, the rear plastic film 15 is annular, has the same outer diameter as the metallic residual sheet 13, and is superposed with the metallic sheets 13 to cover the entire peripheral part of the rear surface of the metallic residual sheet 13. The metallic residual sheet 13 is formed of a magnetic material, such as silicon steel containing 1% of silicon. The plastic films 14 and 15 are formed of abrasion-resistant resinous material. The plastic films 14 and 15 have a thickness of several microns. They are therefore difficult to assemble. In order to keep them in the desired shape during assembly, plastic rings 14a and 15a are bonded to the edges of the plastic films 14 and 15.
Furthermore, as shown in FIG. 3, being a partial sectional view of a main part of the print head, lubricating fluid such as grease 20 for lubrication is applied on the tip 8e of the fulcrum member 8a and the core 9 which face the radial parts 3a to prevent the wear of these parts.
Because of the repeated application of heat and vibration accompanying the printing operation, grease 20 may penetrate between the rear film 15 and the residual sheet 13, and then between the residual sheet 13 and the front film 14, following the path indicated by arrows Pa and Pb in FIG. 3. If a pinhole is formed in the front film 14 due to wear, the grease 20 may ooze out as indicated by arrow PC, and enter the space between the plate spring 3 and the front film 14, and may adhere to the plate spring 3. Thus, a sticking due to grease occurs. The sticking will act as a load when the armature 2 swings during printing. Accordingly, when such sticking occurs, the pixels are not fully printed.
The metallic residual sheet is formed of a ferromagnetic material, and very easily rusts. The metallic residual sheet is therefore given a rust-proof treatment. But the required accuracy of the thickness of the residual sheet is very high, and when the variation in thickness of the metallic residual sheet is on the order of several microns, the operation characteristic of the print head are adversely affected. For this reason, it is difficult to perform a complete rust-proof treatment. When, therefore, the humidity is high, or the residual sheet is touched by hand during assembly of the print head, leaving a fingerprint, the life of the print head is shortened.